imageNumber = 10 + Math.round(Math.random() * 6);

document.write('<style>');
document.write('td.RightSideBackground {');
document.write('  background-image: url(http://images.harc.edu/Sites/HoustonRegionalForest/RightSide/Background' + imageNumber + '.jpg);');
document.write('}');
document.write('</style>');

/*
if(document.styleSheets[2].rules)
  document.styleSheets[2].rules[0].style.backgroundImage = "url(http://images.harc.edu/Sites/HoustonRegionalForest/RightSide/Background" + imageNumber + ".jpg)";

if(document.styleSheets[2].cssRules)
  document.styleSheets[2].cssRules[0].style.backgroundImage = "url(http://images.harc.edu/Sites/HoustonRegionalForest/RightSide/Background" + imageNumber + ".jpg)";
*/